I. Field of the Invention
The present invention relates generally to detection of data rates in communication signals. More specifically, the present invention relates to a system and method for detecting when zero-rate data frames in communication signals are received at a communications receiver.
II. Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
A variety of multiple access communications systems and techniques have been developed for transferring information among a large number of system users. These techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) spread-spectrum techniques, the basics of which are well known in the art. The use of CDMA techniques in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307, which issued Feb. 13, 1990, entitled xe2x80x9cSpread Spectrum Multiple Access Communication System Using Satellite or Terrestrial Repeaters,xe2x80x9d and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997, entitled xe2x80x9cMethod and Apparatus for Using Full Spectrum Transmitted Power in a Spread Spectrum Communication System for Tracking Individual Recipient Phase Time and Energyxe2x80x9d, which are both incorporated herein by reference.
The above-mentioned patent documents disclose multiple access communications systems in which a large number of generally mobile or remote system users employ user terminals or mobile stations to communicate with other system users or users of other connected systems, such as a public telephone switching network. The user terminals communicate through gateways and satellites using CDMA spread-spectrum type communications signals.
Communications satellites form beams which illuminate xe2x80x9cspotsxe2x80x9d produced by projecting satellite communications signals onto the Earth""s surface. A typical satellite beam pattern comprises a number of beams arranged in a predetermined coverage pattern. Typically, a number of CDMA channels (also referred to as sub-beams), each occupying a different carrier frequency, are transmitted in a beam.
In a typical spread-spectrum communications system, a set of preselected pseudo-random noise (PN) code sequences is used to spread information signals over a predetermined spectral band prior to modulation onto a carrier frequency for transmission. PN spreading, a method of spread-spectrum transmission that is well known in the art, produces a signal for transmission that has a bandwidth much greater than that of the underlying data signal being transmitted. In a forward communications link (that is, in a communications link originating at a gateway and terminating at a user terminal), different PN spreading codes or binary sequences are used to distinguish signals transmitted by one or more gateways over different beams. These PN codes are shared by all communications signals within a given sub-beam.
In a typical CDMA spread-spectrum system, different channelizing codes are used to discriminate between signals intended for different user terminals within a sub-beam on the forward link. That is, a unique orthogonal code channel is provided for each user terminal on the forward link by using a unique xe2x80x9cchannelizingxe2x80x9d orthogonal code. Walsh functions, or equivalent codes, are generally used to implement the channelizing codes.
Typical CDMA spread-spectrum communications systems, such as disclosed in U.S. Pat. No. 4,901,307, contemplate the use of coherent modulation and demodulation. In mobile communications systems using this approach, a xe2x80x9cpilotxe2x80x9d carrier signal (hereinafter referred to as a xe2x80x9cpilot signalxe2x80x9d) is used as a phase reference for demodulation. When this approach is used on the forward link, a pilot signal, which typically contains no data modulation, is transmitted by a gateway throughout a coverage region. A single pilot signal is typically transmitted by each gateway for each beam on each frequency (sub-beam) used. These pilot signals are shared by all user terminals receiving signals from the gateway.
In the forward link, pilot signals are used by user terminals to obtain initial system synchronization and as a time, frequency, and phase reference for demodulating other signals transmitted by the gateway. Phase information obtained from a pilot signal is used as a carrier phase reference for coherent demodulation of other system signals or traffic signals. Since all forward link signals on a sub-beam are transmitted synchronously, this technique allows all traffic signals on a sub-beam to share a common pilot signal as a phase reference, providing for a less costly and more efficient signal acquisition or tracking mechanism.
When a user terminal is not involved in a two-way communications session (that is, the user terminal is not receiving and transmitting traffic signals simultaneously), the gateway can convey information to that particular user terminal using a signal channel known as a paging channel. For example, when a call has been placed to a particular mobile station or user terminal, phone, the gateway alerts the mobile station by means of a message on the paging channel. Paging channel messages are also used to distribute information about traffic channel assignments, access channel assignments, system configuration, and the like.
A user terminal can respond to a paging channel message by sending an access channel message over the reverse link (that is, the communications link originating at the user terminal and terminating at the gateway). The access channel is also used by a user terminal when it originates a call, that is requests that a communication link be established.
In a preferred embodiment, data on the forward and reverse links are transmitted at a variable rate. During conversations as well as data transmissions, there are often pauses and periods of intermittent activity that generate little information to be transmitted. On a duplex communication link, such as voice telephone links, often one party is speaking and the other is listening. For voice telephone links, therefore, one of the two links is idle almost half of the time, and can be transmitted using a lower data rate. Additionally, during pauses or periods of intermittent activity, data on forward and reverse links are transmitted at a lower rate. This lower data rate can approach or be equal to zero, that is, no data, in some situations.
Variable rate data transmission results in many benefits in a multiple access system. Detection of the transmitted signal by the receiver depends on signal energy. By decreasing the rate of data transmission, signal energy can be maintained with a lower average transmission power. This is very useful for power limited applications, such as in mobile transceivers. In such cases, reducing the average transmitter output power increases the time before power sources such as batteries must be recharged or replaced, and overall battery life. Additionally, lower power reduced rate transmission reduces the interference and background noise seen by other receivers on the sub-beam. Since transmitters are power controlled to maintain a constant signal energy to noise power ratio at the receiver, lower background noise power results in reduced signal energy requirements. Lower background noise power, therefore, allows other transmitters on the sub-beam to transmit at lower power. Reduced interference may also allow desired additional user capacity in a communication system.
What is needed, therefore, is a system and method that allow the transmission and detection of zero-rate data transmission on the forward and reverse communication links.
The present invention satisfies the above-mentioned needs by providing a system and method for the detection of zero-rate communication frames on forward and reverse communication links. The method for detecting a zero-rate traffic channel, comprises the steps of receiving a pilot channel frame and a traffic channel frame, measuring an energy of the traffic channel frame, measuring an energy of the pilot channel frame, and comparing the pilot channel frame energy to the traffic channel frame energy to obtain a metric. Generally, the frames are compared by computing a ratio of the traffic channel to the pilot channel frame energies. The metric is compared with a predetermined threshold to obtain a result, with the predetermined threshold being based on the probability of detection of a transmitted zero-rate traffic channel frame and the probability of erroneously detecting a zero-rate traffic channel frame. This is generally accomplished by comparing the magnitude of the metric to the magnitude of the predetermined threshold. Whether or not a zero-rate traffic channel frame has been received is indicated based on the result of the metric comparison. A zero-rate traffic channel frame is indicated as being received when the magnitude of the predetermined threshold exceeds the magnitude of the metric.
The system for detecting a zero-rate traffic channel comprises means for receiving the pilot and traffic channel frames, means for measuring an energy of the traffic channel frame, means for measuring an energy of the pilot channel frame, and means for comparing the pilot channel frame energy to the traffic channel frame energy to obtain a metric. Generally, this is accomplished using means for computing a ratio of the traffic and pilot channel frame energies. Means for comparing the metric with a predetermined threshold is used to obtain a comparison result, with the predetermined threshold being based on the probability of detection of a transmitted zero-rate traffic channel frame and the probability of erroneously detecting a zero-rate traffic channel frame. This is generally accomplished by using a means for comparing the magnitude of the metric to the magnitude of the predetermined threshold. As before, whether or not a zero-rate traffic channel frame has been received is indicated based on the result of the metric comparison. Means for indicating whether a zero-rate traffic channel frame has been received based on the result is used to make this indication. A zero-rate traffic channel frame is indicated as being received when the magnitude of the predetermined threshold exceeds the magnitude of the metric.